Bottom Line:
It was found that the biosynthesized PPGs had higher scavenging abilities when compared to ascorbic acid, the reference compound, while their antioxidant activities were found similar to that of natural PPGs.Moreover, density functional theory (DFT) calculations were used to determine that the PPGs antioxidant mechanism proceeds through a sequential proton loss single electron transfer (SPLET).The enzymatic process reported in this study is an efficient and versatile route to obtain PPGs from different phenylpropanoid acids, sugars and phenolic alcohols.

ABSTRACTPhenylpropanoid glycosides (PPGs) are natural compounds present in several medicinal plants that have high antioxidant power and diverse biological activities. Because of their low content in plants (less than 5% w/w), several chemical synthetic routes to produce PPGs have been developed, but their synthesis is a time consuming process and the achieved yields are often low. In this study, an alternative and efficient two-step biosynthetic route to obtain natural PPG analogues is reported for the first time. Two galactosides were initially synthesized from vanillyl alcohol and homovanillyl alcohol by a transgalactosylation reaction catalyzed by Kluyveromyces lactis β-galactosidase in saturated lactose solutions with a 30%-35% yield. To synthesize PPGs, the galactoconjugates were esterified with saturated and unsaturated hydroxycinnamic acid derivatives using Candida antarctica Lipase B (CaL-B) as a biocatalyst with 40%-60% yields. The scavenging ability of the phenolic raw materials, intermediates and PPGs was evaluated by the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) method. It was found that the biosynthesized PPGs had higher scavenging abilities when compared to ascorbic acid, the reference compound, while their antioxidant activities were found similar to that of natural PPGs. Moreover, density functional theory (DFT) calculations were used to determine that the PPGs antioxidant mechanism proceeds through a sequential proton loss single electron transfer (SPLET). The enzymatic process reported in this study is an efficient and versatile route to obtain PPGs from different phenylpropanoid acids, sugars and phenolic alcohols.

pone-0020115-g005: Structural representation of compounds from Table 1 and their deprotonated species are shown.When known, the experimental pKa values are also included.

Mentions:
To analyze the possible reaction mechanism to scavenge free radicals of the galactosylated PPG precursors as well of the new PPG analogs, compounds 3 and 10 were theoretically studied. Also for comparison, compounds 5, 6, 7 and 8 were analyzed. Optimized structures and theoretical results concerning the optimization are included in the Materials and Methods S1. According with the results given in Table 1, caffeic (7) and ferulic acids (8) have lower free radical scavenging activities than their corresponding saturated derivates (5>7>6>8). It is well known that the SPLET mechanism explains the reaction of phenols with DPPH•, nevertheless both mechanisms (ET and SPLET) were studied for compounds 3 and 10 and the theoretical results compared to the experimental values. It is important to note that according to their pKa values, compounds 5, 6, 7 and 8 are deprotonated in the experimental conditions (pH = 7) as shown in Figure 5. For this reason and to analyze the ET capacity of these compounds, RIE and REA were calculated for 3, (5-H) −1, (6-H) −1, (7-H) −1, (8-H) −1 and 10.

pone-0020115-g005: Structural representation of compounds from Table 1 and their deprotonated species are shown.When known, the experimental pKa values are also included.

Mentions:
To analyze the possible reaction mechanism to scavenge free radicals of the galactosylated PPG precursors as well of the new PPG analogs, compounds 3 and 10 were theoretically studied. Also for comparison, compounds 5, 6, 7 and 8 were analyzed. Optimized structures and theoretical results concerning the optimization are included in the Materials and Methods S1. According with the results given in Table 1, caffeic (7) and ferulic acids (8) have lower free radical scavenging activities than their corresponding saturated derivates (5>7>6>8). It is well known that the SPLET mechanism explains the reaction of phenols with DPPH•, nevertheless both mechanisms (ET and SPLET) were studied for compounds 3 and 10 and the theoretical results compared to the experimental values. It is important to note that according to their pKa values, compounds 5, 6, 7 and 8 are deprotonated in the experimental conditions (pH = 7) as shown in Figure 5. For this reason and to analyze the ET capacity of these compounds, RIE and REA were calculated for 3, (5-H) −1, (6-H) −1, (7-H) −1, (8-H) −1 and 10.

Bottom Line:
It was found that the biosynthesized PPGs had higher scavenging abilities when compared to ascorbic acid, the reference compound, while their antioxidant activities were found similar to that of natural PPGs.Moreover, density functional theory (DFT) calculations were used to determine that the PPGs antioxidant mechanism proceeds through a sequential proton loss single electron transfer (SPLET).The enzymatic process reported in this study is an efficient and versatile route to obtain PPGs from different phenylpropanoid acids, sugars and phenolic alcohols.

ABSTRACTPhenylpropanoid glycosides (PPGs) are natural compounds present in several medicinal plants that have high antioxidant power and diverse biological activities. Because of their low content in plants (less than 5% w/w), several chemical synthetic routes to produce PPGs have been developed, but their synthesis is a time consuming process and the achieved yields are often low. In this study, an alternative and efficient two-step biosynthetic route to obtain natural PPG analogues is reported for the first time. Two galactosides were initially synthesized from vanillyl alcohol and homovanillyl alcohol by a transgalactosylation reaction catalyzed by Kluyveromyces lactis β-galactosidase in saturated lactose solutions with a 30%-35% yield. To synthesize PPGs, the galactoconjugates were esterified with saturated and unsaturated hydroxycinnamic acid derivatives using Candida antarctica Lipase B (CaL-B) as a biocatalyst with 40%-60% yields. The scavenging ability of the phenolic raw materials, intermediates and PPGs was evaluated by the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) method. It was found that the biosynthesized PPGs had higher scavenging abilities when compared to ascorbic acid, the reference compound, while their antioxidant activities were found similar to that of natural PPGs. Moreover, density functional theory (DFT) calculations were used to determine that the PPGs antioxidant mechanism proceeds through a sequential proton loss single electron transfer (SPLET). The enzymatic process reported in this study is an efficient and versatile route to obtain PPGs from different phenylpropanoid acids, sugars and phenolic alcohols.